CameraITS: relax several test threshold

test_linearity: allow an average error of 2/255.
test_ae_precapture_trigger: allow up to 24 frames.
Also remove test_black_white and test_locked_burst from not yet
mandated list since known issues are resolved.

Bug: 18644101
Change-Id: Id85be9f00f4e2e9118ef9553b50a86b772ae54cb